Directional couplers



5 Sheets-Sheet 1 ENERGY REFLECTED II II Coupling Element D. J. NIGG DIRECTIONAL COUPLERS SOURCE Um I Dec. 5, 1961 Filed June 4, 1959 A lfarney omscr ENERGY INVENTOR. Donald J. Nigg LOAD Dec. 5, 1961 D. J. NIGG DIRECTIONAL COUPLERS 5 Sheets-Sheet 2 Filed June 4, 1959 n I 2 2 I/ 2 48 48 U C G M M M 0 0 5 0 E 0/0 M Ml J A INVENTOR. Donald 'J. 'Nigg Attorney 1961 D. J. NIGG 3,012,210

DIRECTIONAL COUPLERS Filed June 4. 1959 5 Sheets-Sheet 3 INVENTOR. Donald J. Nl'gg Attorney Dec. 5, 1961 D. J. NIGG 3,012,210

DIRECTIONAL COUPLERS Filed June 4, 1959 5 Sheets-Sheet 4 IIIII IIIIIII Fig. 9

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INVENTOR. Donald J. Nigg A ftorney Patented Dec. 5, 15261 3,012,219 DHKEQTIONAL COUPLERS Donald J. Nigg, irairie Village, Kana, msignor to the United States of America as represented by the United States Atamic Energy Commission Filed .iune 4, 1959, Ser. No. 318,234 Claims. (Cl. 3333-40) This invention relates to apparatus which may be employed for coupling energy flowing in a primary transmission line into a secondary transmission line in a manner such that the direction of flow in the secondary line is responsive to the direction of energy flow in the primary transmission line. Devices of this type have become known as directional couplers and are generally employed in connection with various known types of high frequency transmission lines, such as Wave guide, two-wire line, and coaxial cable.

One general type of directional coupler is that which employs coupling elements which are arranged between the primary and secondary lines and are spaced a quarter wave length apart in the direction of energy flow. Couplers of this general type have become known as quarterwave directional couplers. The usual coupling elements employed are only nominally either inductive of capacitive, whereas purely inductive or capacitive coupling elements are'actually necessary to attain ideal performance.

In an ideal quarter-Wave directional coupler, only the coupled direct energy will appear at one of its secondary terminals and only the coupled reflected energy will appear at the other secondary terminal.

Prior art quarter-wave directional couplers, particularly the T.E.M. mode side-by-side types in miniaturized versions have not attained this ideal performance because the necessary proximity of the primary and secondary transmission lines or the very nature of the coupling elements themselves invariably has resulted in some appreciable degree of an undesired combination type of coupling, i.e., both inductive and capacitive, and in a consequent reduced directivity.

It is a principal object of this invention to provide a directional coupler of superior electrical performance, particularly useful in the microwave frequency range below 4060 megacycles.

Another object of the invention is to provide anew and improved transmission line coupling means.

Still another object of the present invention is to provide improved coupling means particularly useful with transmission lines wherein the transverse electromagnetic mode of transmission is used.

A further object of the invention is to provide an improved directional coupling means of relatively simple and inexpensive construction particularly adaptable to miniaturization.

A still further object of the invention is to provide an improved directional coupler means permitting a high degree of design flexibility.

Another object of the invention is to provide an improved directional coupler means which lends itself to broad-banding.

Still another object of the invention is to provide an improved directional coupler which may be constructed in accordance with either stripline or Microstrip principles.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiments about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

A preferred embodiment of the invention has been chosen for purposes of illustration and description. The

preferred embodiment illustrated is not intended to be exhaustive nor to limit the invention to the precise form disclosed. It is chosen and described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular use contemplated.

In the accompanying drawings:

FIG. 1 is a schematic diagram of a conventional quarter-Wave type directional coupler;

FIG. 2 is a side view showing a preferred embodiment of the invention and having a portion broken away;

FIG. 3 is an exploded view in perspective of the device shown in FIG. 2;

FIG. 4 is a plan view showing the relationship of the stripline conductors of the device;

FIG. 5 is a diagram indicating approximate dimensions which the stripline array of FIG. 4 may take for certain frequencies;

FIG. 6 is a plan view showing a single stripline conductor of modified form;

FIGS. 7a-7d are plan views similar to FIG. 4 which demonstrate the design flexibility permitted by the invention;

FIG. 8 is a plan view of aportion of a printed circuit employing a single ground plane;

FIG. 9 is a sectional view taken along line 99 of FIG. 8;

FIG. 10 is a plan view of stripline conductors in a broadbanded modification of the device; and

FIG. 11 is a plan view of second stripline embodiment of the invention wherein the primary ground plate has been removed.

The quarter-wave type of directional coupler will first be discussed in general and idealized terms with particular reference to FIG. 1. A primary transmission line comprised of energy source branch 11 and load branch 13 is coupled to a secondary transmission line comprised of reflected energy branch 12 and direct energy branch 14 by means of coupling elements No. l and No. 2. Coupling element No. l is joined to the primary and secondary lines at junctions A and D respectively, and coupling element N0. 2 is joined to the primary and secondary lines at junctions B and C respectively. Junctions A and B are spaced apart by a quarter-wave length along the primary as are junctions C and D along the secondary. In operation, two equal quanta of the wave energy entering source branch 11 in transit to load branch 13 are diverted or coupled into the secondary transmission line by the coupling elements. These two equal Wave energy quanta reach the direct energy branch 14 of the secondary by two separate paths of equal length (namely ABC and ADC) at the same time or in phase and add to produce a wave energy output. I

Equal wave energy quanta leaving primary junction A also have two separate paths (namely ABCD and AD which difier in length by one-half a wave length) by which they can reach junction D for flow into reflected energy branch 12. However, since the wave energy quantum coupled at coupling element No. 2 must travel one-half a wave length further than the wave energy quantum coupled at coupling element No. 1, it necessarily arrives at junction D out of phase with the latter wave energy quantum to effect a wave cancellation which results in zero output at branch 12. Thus in an ideal directional coupler with energy in transit from source to load, only the direct energy branch of the directional coupler will have an output. When the transmission direction in the primary is reversed as when Wave energy is reflected by the load back to the source, the directional coupler will reverse its operation and respond so that an output will appear in the secondary only at the reflected energy branch 12. As has been pointed out, the miniaturized T.E.M. side-by-side quarter-wave couplers of the prior art have not attained this ideal of performance.

Described generally, the device embodying the present invention as shown in FIGS. 2-11 comprises a quarterwave type directional coupler of shielded stripline or sandwich construction wherein the coupling elements are substantially pure capacitances.

With further reference to FIGS. 2-ll of the drawings, the invention is shown embodied in a directional coupling device, generally indicated at in FIGS. 2 and 3, coupling a primary coaxial transmission line comprised of source branch 11 and load branch 13 to a secondary coaxial transmission line comprised of reflected energy branch 12. and direct energy branch 14. A uniform system of reference numerals denoting various transmission line branches is used throughout the present disclosure.

As may be best seen from FIG. 3, the device 10 is comprised generally of five stacked layers which may be fastened together by bolts or other suitable means. Primary and secondary ground plates, 21 and 22 respectively, are the outermost layers. Each has two suitably located conductor openings therethrough which are individually rimmed by bosses on the outer ground plate surface. Each of these openings permits the insulated center conductor of a coaxial transmission line branch to pass through the ground plate while the associated boss provides a convenient location for connecting the coaxial shielding of the branch to that ground plate.

Primary and secondary stripline cards 31 and 32 respectively which are respectively associated with ground plates 21 and 22, comprise the next outermost layers of the device. These may be made of glass-fiber-reinforced tetra fluoroethylene (Teflon), although any other suitable insulating material may be used. Each of the strip line cards also has two conductor openings which are located so that, when a card and its associated ground plate are in proper stacked relation, each conductor opening in the card is aligned with a ground plate conductor opening, thereby enabling two coaxial transmission line branch center conductors to pass through an associated ground plate and card pair.

A stripline conductor is carried on one side of each stripline card (primary stripline conductor 41 on card 31 and secondary stripline conductor 42 on card 32) in such a way that the stripline conductor is insulated by the card from the ground plate associated with that card; that is, it is disposed on the card surface remote from the ground plate. Primary stripline conductor 41 has perforated terminal ends 43 and 45 spaced so that the center conductors of coaxial branches 11 and 13 respectively may extend therethrough and be connected thereto, as by soldering. Secondary stripline conductor 42 has similar ends 44 and 46 for similar connection to the center conductors of branches 12 and 14 respectively. The stripline conductors of the illustrated embodiment may be made of metal foil and secured in place on the cards at assembly by the solder joints of the electrical connections. While shown as being separate parts, the conductors may be printed on the cards by photo etching or any other process of producing a conducting element on an insulated surface.

A thin dielectric spacer 50 comprises the central layer of the device and serves to space and insulate the primary stripline conductor 41 from the secondary stripline conductor 42 on opposed faces of cards 31 and 32 respectively when all the elements of the device are properly assembled in stacked relation. Although the dielectric spacer 50 is preferably made of substantially paper-thin rifluorochlorethylene (Kel-F) sheeting, any other dielectric material having suitable properties may be used. It should be noted that a vacuum or various gasses may serve as a satisfactory dielectric medium when separate mechanical spacing means are employed.

As has been indicated, electrical connection of the device to primary and secondary coaxial transmission lines is as follows: The coaxial shield of source branch 11 and load branch 13 is each connected to the primary ground plate 21 and the insulated center conductors of each pass through a conductor opening in the ground plate and on through an aligned conductor opening in primary stripline card 31 to a point where each center conductor is connected respectively to ends 43 and 45 of stripline conductor 41. Similarly, the reflected energy branch 12 and the direct energy branch 14 of the secondary coaxial transmission line have their shielding connected to secondary ground plate 22 and their center conductors respectively connected to ends 44 and 46 of secondary stripline conductor 42. It thus becomes apparent that the center conductors of the primary transmission line branches are electrically joined by primary stripline conductor 41 as are the center conductors of the secondary transmission line branches by secondary stripline conductor 42. It should be noted that when the device is tightly assembled the two stripline conductors are generally held in very close dielectrically spaced relationship, being separated by only the thin dielectric spacer 50.

Although not necessary to the operation of the device, the primary and secondary ground plates are shown electrically connected by means of the bolts which, as has been pointed out, also function to hold the elemental layers of the device together. Such electrical interconnection of the ground plates afiords a better transition from coaxial to stripline transmission lines. When such improved transitions are desired, a number of fasteners may be arrayed in a roughly circular pattern around each transition.

Although the device has been described for use with a secondary transmission line comprised of both reflected and direct energy branches, it is apparent that a suitable energy attenuator may be incorporated in the device or otherwise substituted for either secondary branch. This would, in effect, reduce the secondary to one branch which, of course, would produce an output only from energy flowing in a particular direction in the primary transmission line.

A more particular description of the relationship between the primary and secondary stripline conductors of the device follows. For this purpose, attention is directed to FIG. 4 wherein the stripline conductors 41 and 42 of one embodiment of the device are shown to be of somewhat dog-legged form and together form a closed array as viewed in plan. The stripline conductors are arranged such that stripline conductor 41 crosses over stripline: conductor 42 at two places to form capacitive areas 51 and 52 which are defined in extent by the mutual projection or overlying portions of the two stripline conductors.

Portions of the two stripline conductors lying within capacitive areas 51 and 52 and spaced by previously de' scribed dielectric spacer 50 comprise the capacitive cou-- pling elements of the device. It should be noted that these coupling elements have no leads in the usual sensefor joining to the primary and secondary stripline conductors. Another way of viewing this latter point is. that due to the unique arrangement of the stripline conductors, the capacitive coupling element leads have been reduced substantially to zero length which, in turn, substantially eliminates coupling capacitor lead inductance which has been troublesome in prior art devices.

It is to be further noted that the centers of capacitive areas 51 and 52 are spaced apart a total stripline conductor center-line distance of approximately one-quarter wave length and that each stripline conductor extends outwardly beyond the capacitive areas a short distance sufficient to isolate the stripline ends from these areas. Since these stripline extensions function as electrical extensions of the center conductors of the primary and secondary lines, they are for convenience denoted by numerals 11, 12, 13, and 14 in accordance with the uniform transmission line branch reference number system of this disclosure.

The angular relationship of the stripline conductors in the neighborhood of capacitive areas 51 and 52 is of primary importance in achieving the superior electrical performance of the device. The desired relationship is that the stripline conductor centerlines form substantially a right angle as they cross. This places each stripline conductor in magnetic coupling null relationship with respect to the other and thereby substantially eliminates undesirable inductive coupling effects. Hence, essentially pure capacitive coupling is achieved with the device.

The general operation of the present device is substantially the same as that of the previously described idealized quarter-wave directional coupler of FIG. 1. If the present device counterparts of junctions A and D and coupling element No. 1 of the ideal device are thought of as being concentrated at capacitive area 51 and junctions B and C and coupling element No. 2 are similarly thought of as being concentrated at capacitive area 52, the previous description of the operation of the idealized coupler may be applied to the present device as shown in FIG. 4.

FIGURE 5 shows the manner in which a Teflonglass card insulated stripline array such as is shown in FIG. 4 varies in size according to the approximate center frequency of the band for which it is designed. It is evident that the design wave length value generally controls the minimum area dimensions of the various layers of such a device designed for a particular frequency, and therefore generally controls its overall size.

The coupling and directivity characteristics of the device may be altered by substituting difierent dielectric spacers 50 of varying thicknesses or dielectric constants. For example, the use of a thinner dielectric spacer will result in tighter coupling and in increased directivity. In addition, the coupling characteristics of the device may be altered by changing the size of the mutual projection or overlying portions of each stripline conductor. As an example of this technique, FIG. 6 shows quarterwave spaced pad enlargements 60 on a stripline conductor. When two such modified conductors are properly positioned the size of the capacitive coupling areas is increased, which, in turn, results in tighter coupling. This latter technique of varying areas is especially useful at the lower frequencies where the dielectric spacer variation technique may meet with practical minimum thickness limitations.

Although the relation of the stripline conductors has been described in terms of stripline conductors embodying a somewhat dog-legged form, such a form is merely an exemplification of a number of configurations that the stripline conductors may take, a few of which are shown in FIGS. 711-7a. The only requirements of a workable stripline conductor configuration are that it is adapted to cross over another at two places which define substantially equal capacitive areas having substantially equal actual energy coupling value in the particular circuit and which are spaced a quarter-wave length apart along the centerline of each conductor and that such centerlines include substantially a right angle in the neighborhood of such crossovers. Although the various illustrations all show somewhat similar configurations being used together, dissimilar configurations may be used so long as the above requirements are adhered to.

While the preferred embodiment illustrates transition from coaxial primary and secondary transmission lines to a stripline directional coupler configuration, it is obvious that the coupler may directly couple primary and secondary printed circuit transmission lines, i.e., the coupler configuration may be an integrally incorporated elemental part of a more complex board circuit of either type.

FIGURES 8-9 show the directional coupler of the invention as an element of larger more complex printed circuit (not shown) which employs only a single ground plane. As such, the coupler is comprised of a dielectric base card 64 having etched primary and secondary transmission lines having branches (11 and 13) and (12 and 14) respectively on one card face and a conducting ground plane 65 on the other. As etched (shown in the left hand portion of FIG. 8), one of the transmission lines is discontinuous where it would otherwise intersect the other transmission line. These discontinuities are then bridged by suitable conducting crossover straps 66 which are soldered or otherwise connected to the discontinuous segments to electrically complete the discontinuous line and to establish capacitive crossover areas. A thin dielectric shim 67 is interposed between the crossover strap and that portion of the other circuit over which it crosses. Thus it is seen that in a single-groundplane printed circuit application a capacitive coupling unit is comprised of a portion of a continuous transmission line, a thin dielectric shim, and a crossover strap. The operation of such a quarter-wave directional coupler as an element of a single-ground-plane printed circuit is substantially as previously described for an ideal coupler. It should be noted that the requirements of a workable single-ground-plane printed circuit coupler configuration and the degree of flexibility in such design are the same as previously described for stripline conductors.

The previously described crossover straps 66 may be conveniently etch-formed (in otherwise unoccupied circuit board or card locations such are indicated at 68 and 69 in FIG. 8) during the circuitry etching process. After such forming, the strap may be peeled off the board or card and soldered in place to bridge the aforementioned discontinuities.

To further illustrate the flexibility of the device, FIG. 10 shows a pair of stripline configurations constituting a multiple path coupler array having broad band characteristics. As illustrated, there are three capacitive coupling areas, namely 73, 74, and 75, which vary in area in accordance with the well known general principles of the binomial coeflicient method of broadbanding directional couplers.

FIGURE 11 shows another embodiment of the invention incorporating means whereby the frequency response may be tuned over a wide range. For convenience the primary ground plate is not shown. Reference numerals used to denote similar parts in the preferred embodiment of FIG. 3 are used where applicable in FIG. 11. Suitable slots 81 and 82 respectively in the primary and secondary layers of the device provide passageways for fasteners to pass through and hold the various layers of the device in stacked relation. In addition, the slots and fasteners enable the lateral shifting of the primary and secondary elements with respect to each other in a direction so as to vary the quarter-wave length spacing of the capacitive coupling areas and tune the device to various frequencies, but at the same time enablins the maintenance of angle 0 at its optimum value of degrees.

it is apparent that the invention provides a Wide latitude for the designer, especially in m niaturization, and also enables devices embodying the invention to be readily adjustable in a number of different ways. In addition it is seen that devices embodying the invention achieve superior directional coupling performance primarily as a result of a unique arrangement of stripline conductor pairs whereby undesired inductive coupling effects are substantially eliminated.

As various changes may be made in the form, construction and arrangement of the parts herein without departing from the spirit and scope of the invention and Without sacrificing any of its advantages, it is to be underat a plurality of locations of substantially equal spacing along each other and with their centerlines defining substantially right angles in the neighborhoods of their crossings, and a layer of solid dielectric material intermediate said first and second conductors spacing them from each other.

2. The device of claim 1 wherein said conductors are provided with enlargements at said crossing locations.

3. The device of claim 1 wherein at least one of said conductors is movable with respect to the other to vary the effective length of said equal spacing.

4. A directionai coupler comprising a pair of spaced apart ground plane members, a pair of substantially planar conductors intermediate said ground plane members and spaced therefrom each having a concave-like configuration at one edge thereof and a convex-like configuration at an opposite edge thereof, said conductors being disposed With said concave-like edges toward each other and the conductors crossing each other at a plurality of locations of substantially equal spacing along each other and centerlines of the conductors defining substantially right E; angles in the neighborhoods of their crossings, and a layer of solid dielectric material intermediate said conductors spacing them from each other.

5. A directional coupler having primary and secondary sections each comprised of a ground plane plate, an insulating card next inwardly of said plate, and an inner conductor next inwardly adjacent said card, said primary and secondary sections being spaced by a solid dielectric layer intermediate said inner conductors, said inner conductors being shaped and arranged that one crosses the other at locations of substantially equal spacing along each other in a manner such that their centerlines define substantially right angles within the immediate neighborhood of said crossings and such that their mutual proiection portions comprise dielectrically spaced capacitive coupling elements, said ground plane plates having means for grounding the shielding of associated transmission line branches and said sections being penetrated at said means to enable the center conductor of an associated branch line to' pass through said section and connect with an end of the inner conductor associated with the section penetrated thereby.

References Cited in the file of this patent UNITED STATES PATENTS 

